Flame-proofed polystyrene foamed materials

ABSTRACT

Flame-retardant foam slabs are based on styrene polymers wherein from 1 to 12% by weight of expanded graphite and also, if desired, from 1 to 12% by weight of a phosphorus compound are present as flame retardants.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to extruded foam slabs with a density of from 20to 200 g/l and with a cross-sectional area of at least 50 cm², and basedon styrene polymers equipped with halogen-free flame retardants.

2. Description of the Background

Extruded polystyrene foams (XPS) are widely used to insulate buildingsand components of buildings. For this application they have to beflame-retardant. The flame retardants usually used to render extrudedpolystyrene foams flame-retardant comprise halogens. For environmentalreasons the use of halogens in flame retardants should be avoided.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide extruded polystyrenefoams which achieve the fire classification B2 required for buildingindustry applications and which have been produced without the use ofhalogen-containing flame retardants.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

We have found that this object is achieved by using, as flame retardant,from 1 to 12% by weight, preferably from 3 to 8% by weight, of expandedgraphite, if desired together with from 1 to 12% by weight, preferablyfrom 3 to 8% by weight, of a phosphorus compound, based in each case onthe styrene polymer.

The literature describes expanded graphite in combination with redphosphorus and/or with phosphorus-containing compounds as a flameretardant for compact polystyrene. In previous experiments, however, ithas been found that halogen-free flame retardants which can be used forcompact polystyrene cannot be used for foam production, since there iseither severe adverse effect on the foaming process or an excessivereduction in the heat resistance of the foam. Surprisingly, however,this is not the case in the present invention.

U.S. Pat. No. 3,574,644 describes the addition of expanded graphite asflame retardant for combustible materials, inter alia for foams in whichthe amount of the expanded graphite to be present is from 20 to 40% byweight. The expanded graphite may either be incorporated into theexpandable material prior to the expansion process or may be used tocoat the expandable material after the expansion process. There is nodescription of XPS slabs with from 1 to 12% by weight of expandedgraphite.

JP-A 03-167 263 describes a polystyrene foam which comprises, as flameretardant, expanded graphite whose surface was completely coated with afilm-forming resin. This coating is described as necessary to avoidcorrosion of processing machinery by the acids always present in theexpanded graphite, but it requires an additional and complicatedoperation. Besides the expanded graphite, the polystyrene foam may alsocomprise conventional flame retardants, e.g. halogenated organicphosphates. The polystyrene foam is preferably prepared by mixingpolystyrene foam beads with an adhesion promoter and with the coatedexpanded graphite. It may also be obtained by extruding a mixture ofpolystyrene with a blowing agent and with the coated expanded graphite.Surprisingly, it has been found that coating of the expanded graphitewith a resin film is not required for XPS slabs in which from 1 to 12%of expanded graphite is present as flame retardant.

The layered lattice structure of graphite permits it to form a specifictype of intercalation compound. In these compounds, which are known asinterstitial compounds, foreign atoms or foreign molecules have beenabsorbed into the spaces between the carbon atoms, sometimes instoichiometric ratios. These graphite compounds, e.g. with sulfuric acidas the foreign molecule, are also produced on an industrial scale andare termed expanded graphite. The density of this expanded graphite isfrom 1.5 to 2.1 g/cm³, and its average particle size is from 20 to 2000μm, in particular from 30 to 1000 μm.

Phosphorus compounds which may be used are halogen-free inorganic ororganic phosphates, phosphites or phosphonates, and also red phosphorus.Examples of preferred phosphorus compounds are triphenyl phosphate,diphenyl cresyl phosphate, ammonium polyphosphate, melamine phosphate,resorcinol diphenyl phosphate and dimethyl methylphosphonate.

The novel foam slabs are produced by extruding and foaming a mixturemade from the styrene polymer, from 3 to 15% by weight of a volatileblowing agent and from 1 to 12% by weight of expanded graphite, andalso, if desired, from 1 to 12% by weight of a phosphorus compound, atabove the melting point of the styrene polymer. The ratio of weights ofexpanded graphite and phosphorus compound is preferably from 20:80 to80:20.

For the purposes of this invention, styrene polymers are polystyrene andstyrene copolymers incorporating at least 80% by weight of styrene.Examples of possible comonomers are α-methylstyrene, ring-halogenatedstyrenes, ring-alkylated styrenes, acrylonitrile, (meth)acrylates ofalcohols having from 1 to 8 carbon atoms, N-vinyl compounds, such asvinylcarbazole, maleic anhydride, or else small amounts of compoundswhich contain two polymerizable double bonds, for example butadiene,divinylbenzene or butanediol diacrylate. The styrene polymers maycomprise from 1 to 20%, based on their weight, of other thermoplasticpolymers, e.g. polyacrylates or polyacrylonitrile, or also dienerubbers.

Blowing agents which may be used are the usual volatile organiccompounds, such as fluorochlorohydrocarbons, fluorohydrocarbons,hydrocarbons, alcohols, ketones and ethers. However, preference is givento halogen-free blowing agents and blowing agent mixtures, e.g.inorganic gases, such as carbon dioxide, nitrogen, argon or ammonia, ifdesired mixed with alcohols, hydrocarbons or ethers.

Preferred blowing agent mixtures are therefore:

a) from 1 to 100% by weight of carbon dioxide,

b) up to 95% by weight of an ether selected from the class consisting ofdimethyl ether, methyl ethyl ether and methyl vinyl ether,

c) up to 60% by weight of an alcohol or ketone with a boiling point offrom 56 to 100° C., and

d) up to 30% by weight of an aliphatic C₃-C₆ hydrocarbon.

Particularly preferred blowing agent mixtures are composed of:

a) from 20 to 90% by weight of carbon dioxide,

b) from 1 to 30% by weight of dimethyl ether,

c) from 0 to 60% by weight of ethanol, and

d) up to 10% by weight of an aliphatic C₃-C₆ hydrocarbon, or of:

a) from 20 to 95% by weight of carbon dioxide,

c) from 80 to 5% by weight of ethanol, and

d) from 0 to 10% by weight of an aliphatic C₃-C₆ hydrocarbon.

Carbon dioxide may also be used on its own.

The amount of blowing agent used, based on the styrene polymer, is from3 to 15% by weight, preferably from 4 to 12% by weight.

Other conventional additives and/or auxiliaries which may be used in thethermoplastic matrix are antistats, stabilizers, dyes, fillers,rathermanous particles, such as carbon black, graphite or metal powders,and/or nucleating agents, in the usual amounts.

All parts and percentages in the following examples are by weight:

A mixture made from 100 parts of polystyrene and the parts of flameretardants given in the table was fed continuously to an extruder withan internal screw diameter of 53 mm, and melted. A blowing agent mixtureof 3.5 parts of C₀ ₂ and 2.1 parts of ethanol was injected continuouslyinto the extruder via an inlet aperture, and mixed into the melt. Thehomogeneously kneaded melt-blowing-agent mixture in the extruder wasthen introduced to a second extruder with a screw diameter of 90 mm andcooled there for a residence time of about 10 min to the temperaturerequired for the foaming process. This cooled melt was then extrudedinto the atmosphere through a slot die of width 70 mm. The resultantfoam was shaped by tooling to give slabs of thickness 30 mm and width150 mm. The foam sections obtained were of the closed-cell type, anduniform and dimensionally stable.

The B2 combustibility of the resultant foam slabs was determined to DIN4102.

The results are given in the table.

TABLE Expanded Red Self- Example graphite phosphorus TPP B2extinguishing 1 — — 10  no no 2 — 10  — no no 3 8 — — yes no 4 6 4 1.5yes yes 5 6 6 — yes yes 6 6 — 6 yes yes

TPP—Triphenyl Phosphate

The expanded graphite used was CALLOTEK 600 from Kropfmühl AG.

We claim:
 1. An extruded foam slab having a density of from 20 to 200g/l and having a cross-sectional area of at least 50 cm², based onstyrene polymers, wherein from 1 to 12% by weight, based on the styrenepolymers, of expandable graphite, having no surface coating, is presentas a flame retardant.
 2. The foam slab of claim 1, which furthercomprises from 1 to 12% by weight, based on the styrene polymers, of ahalogen-free phosphorus compound.
 3. The foam slab of claim 2, whereinthe halogen-free phosphorus compound is red phosphorus, or an organic oran inorganic phosphate, phosphite or phosphonate.
 4. The foam slab ofclaim 2, wherein the halogen-free phosphorus compound is triphenylphosphate, diphenyl cresyl phosphate, melamine phosphate, ammoniumpolyphosphate or diphenyl phosphate.
 5. The foam slab of claim 1,wherein the expandable graphite has a density of from 1.5 to 2.1 g/cm³and an average particle size (longest diameter) of from 20 to 2000 μm.6. The foam slab of claim 1, wherein said expandable graphite is presentin an amount of from 3 to 8% by weight.
 7. The foam slab of claim 5,wherein the expandable graphite has an average particle size of from 30to 1000 μm.
 8. The foam slab of claim 1, wherein said styrene polymerscomprise polystyrene or styrene polymers containing at least 80% byweight of styrene.
 9. The foam slab of claim 1, which meets fireclassification B2 for building industry applications.
 10. A process forproducing the extruded foam slab of claim 1, which comprises the stepsof: a) extruding and foaming a mixture made from styrene polymers, from3 to 15% by weight of a volatile blowing agent and from 1 to 12% byweight of expandable graphite and optionally from 1 to 12% by weight ofa phosphorus compound, based, in each case, on the styrene polymers, atabove the melting point of the styrene polymer.
 11. The process of claim10, wherein a halogen-free blowing agent is used.
 12. The process ofclaim 10, wherein a weight ratio of expandable graphite and phosphoruscompound of from about 20:80 to 80:20 is used.
 13. The process of claim11, wherein said halogen-free blowing agent is an organic compound whichcomprises volatile hydrocarbons, fluorohydrocarbon, alcohols, ketones orethers.
 14. The process of claim 11, wherein said halogen-free blowingagent is an inorganic gas comprising carbon dioxide, nitrogen, argon orammonia.
 15. The process of claim 11, wherein said halogen-free blowingagent is used in an amount of from 3 to 15% by weight based on thestyrene polymers.